It is known that, in general, on an airplane, the vertical stabilizer (that is to say the fixed plane of the vertical empennage of the airplane, which is intended to provide the airplane with navigational stability) and the rudder (that is to say a moveable flap mounted on the vertical stabilizer and which can be maneuvered with a view to altering the direction of the airplane) have dimensions allowing the airplane fully to satisfy the various flight requirements (whether or not these be covered by regulations) such as, in particular, a minimal control speed of the VMC type for failure of one engine.
It is also known that in order to improve airplane performance (fuel consumption, noise levels, etc.) without reducing the payload, constructors have to reduce the mass of the airplane as far as possible, that is to say to reduce the mass of the structure, components, equipment, etc., and in particular the mass of said vertical stabilizer and of said rudder. However, reducing the dimensions of the vertical stabilizer and of the rudder for such purposes mechanically increases the aforementioned VMC speed, to the detriment of airplane performance, because when one engine fails, the minimal speed that has to be achieved is thus increased.